1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to a laminated fabric which is warp knitted/stitch bonded together.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Contemporary fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven or other type base fabric. Additionally, as in the case of fabrics used in the press section, the press fabrics have one or more base fabrics into which has been needled a batt of fine, nonwoven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of the synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back-and-forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing at least one base fabric within the endless loop formed by another, and by needling a staple fiber batt through these base fabrics to join them to one another as in the case of press fabrics. One or more of these woven base fabrics may be of the on-machine-seamable type. This is now a well known laminated press fabric with a multiple base support structure. In any event, the fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long nip press nips has been found to be advantageous over the use of nips formed by pairs of adjacent press rolls. This is so because the longer the time a cellulosic fibrous web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the fibrous web for removal through evaporation in the dryer section.
In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip, which can be five to ten times longer in the machine direction than one between two press rolls, is formed. This increases the so-called dwell time of applied pressure on the fibrous web in the long nip. The result of this new long nip technology has been a dramatic increase in the dewatering of the fibrous web in the long nip when compared to that obtained with conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 5,238,537. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Belts of the variety shown in U.S. Pat. No. 5,238,537 are made by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness at least on the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press. It is specifically this coating which must have a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and fibrous web.
The base fabric of the belt shown in U.S. Pat. No. 5,238,537 may be woven from monofilament yarns in a single- or multi-layer weave, and is woven so as to be sufficiently open to allow the impregnating material to totally impregnate the weave. This eliminates the possibility of any voids forming in the final belt. Such voids may allow the lubrication used between the belt and shoe to pass through the belt and contaminate the press fabric or fabrics and fibrous web.
When the impregnating material is cured to a solid condition, it is bound to the base fabric by a mechanical interlock, wherein the cured impregnating material surrounds the yarns of the base fabric.
Depending on requirements, there is frequently a need to control the depth to which the impregnating material penetrates within the base fabric. This may be desirable either because only one side of the base fabric is to be coated, or because each side is to be coated in a separate operation.
Heretofore, filler or stuffer yarns have been included in the base fabrics for this purpose. Their use has been only partly successful; despite the inclusion of filler or stuffer yarns, some seepage of the impregnating material through the base fabric invariably occurs in a nonuniform manner. Where the base fabric is only to be coated on one side, this presents at least an aesthetic problem, as the uncoated side of the base fabric ultimately appears blotchy and nonuniform due to uneven penetration by the impregnating material.
Because the cured impregnating material is primarily bound to the base fabric by a mechanical interlock, the impregnating material must penetrate to a depth sufficient to ensure that it will to some extent interlock with the base fabric, and will not delaminate readily therefrom. Where both sides of the base fabric are to be coated in separate operations, the impregnating material on each side must be sufficiently bound to prevent delamination. To ensure such an outcome, the impregnating material on the first side to be coated should penetrate uniformly to a depth enabling it to be mechanically bound to the base fabric, while leaving sufficient unimpregnated structure to enable impregnating material applied to the other side to be mechanically bound thereto. If the impregnating material has penetrated too deeply from the first side, that applied to the other side may have too few interlocking sites and may eventually delaminate.
One approach that has been proposed for overcoming this difficulty is to first coat one side of the base fabric with the impregnating material, and to allow that coating to at least partially cure. Then, after inverting (turning inside out) the base fabric, one applies a tie coat to the other side, followed by a coating of the impregnating material. The tie coat provides an additional chemical bond between the coatings on the two sides of the base fabric. The application of the tie coat, however, represents an extra process step and is difficult to control.
In the case of many applications, including woven fabrics, fabrics produced by spirally winding a strip of woven (see U.S. Pat. No. 5,360,656 to Rexfelt (“Rexfelt '656”) or knitted fabric, or laminated fabrics all require some mechanism for either keeping the yarns in place or for joining the fabric together. Typically heretofore needling of staple fiber through a multilayer fabric was utilized to keep it together. Other methods were utilized such as bonding or welding, as taught in EP 0960975.
Placing two or more components together to form a substrate for a paper industry process belt may enhance the belt properties such as providing a better support for the resin coating to penetrate the base and thus prevent delamination of the resin coating layer. It may also control the depth of resin penetration. EP Patent 0960975 attempts to address this issue by producing a substrate comprising two woven base fabrics held together with needled batt like that used in a press fabric. However, the process is costly in terms of material (two separate bases need to be woven), processes (the two bases need to have batt attached by needling), and time needed for each processing step. Other base support substrates for paper industry process belts have been proposed besides woven fabrics. Knitted structures especially have been considered for various reasons. However, no prior art has taught producing a multilayer or multicomponent “laminate,” held together by stitch bonding or knitting, which also provides a degree of control of depth of penetration of resin coating as well as an excellent structure to aid resin/substrate interlock.
The present invention provides an approach towards achieving this end.